After extended periods of hydrocarbon production, many wells suffer from near-borehole formation damage which restricts future production. At such times it is clearly desirable to enhance production and this calls for well intervention to access and/or release the remaining hydrocarbon reserves.
Commonly, well intervention will include sidetracking the existing well—however this generally requires the use of an expensive drilling rig or coiled tubing drilling unit. However, low-cost wells do not economically justify the cost and down-time associated with such well intervention methods, and in any case even with high-cost wells it is extremely desirable to minimise down-time.
Alternatively, it is known to work over by effectively replacing the completion but this is understandably a costly procedure and success is not guaranteed—particularly if the formation has been seriously damaged. Re-perforating, acid washing and other chemical treatments might provide only temporary improvement in production and will also be of limited efficacy if the formation has been damaged and may, particularly with chemical treatments, damage the formation further.
Environmental concerns provide numerous disincentives to employ hydraulic fracturing fluids or other chemical intervention methods, including potential contamination of ground water and effects on air quality. There are also difficulties in predicting how a formation will react to a particular fracturing attempt, and despite lack of conclusive evidence concerns remain regarding inducing earthquakes or ground tremors by injection of fluids into deep wells.
Radial drilling has been employed in well intervention operations, in which coiled tubing and jetting technology is used to drill small diameter wellbores from an existing wellbore to expose clean formation for enhanced hydrocarbon production. However, jetting technology is not always effective in drilling the formation, and coiled tubing units are expensive.
Drilling techniques are preferable to jetting techniques, particularly in well intervention operations. However, problems are inherent in drilling operations—and particularly when sidetracking the existing well or drilling open hole laterals. One such problem lies in the removal of drilling cuttings which would otherwise block production and therefore render drilling operations counterproductive.
In WO 2009/062726, a method of removing cuttings from a workfront of a lateral borehole is described in a series of steps, namely transporting cuttings from the workface to behind the drilling tool, from behind the drilling tool to the junction with the main well, and from the junction to a place of disposal. Various methods of transporting the cuttings from behind the drilling tool are suggested, including withdrawing the drilling tool or using shuttling transport devices. However, the transportation methods disclosed appear to lack mechanical efficiency, and it is suggested that the effectiveness of cuttings removal is correspondingly limited.
GB 2416550 describes a drilling tool that, in a lateral wellbore, employs a first pump to circulate fluid to clear cuttings from the drilling bit and along the wellbore, and a second pump that circulates fluid through the lateral wellbore to transport cuttings out of the lateral wellbore. This relies upon a system of multiple pumps, which in itself is mechanically and (in wireline operations) would be electrically inefficient. Furthermore, it is not clear how the cuttings are subsequently removed from the main wellbore, or how to ensure cuttings in the lateral wellbore are efficiently removed to prevent blockages.
Disclosed in U.S. Pat. No. 7,487,846 is a wireline drilling method in which an electric motor is employed to reverse circulate production fluid through the drill bit to remove drilling cuttings. In the event of blockage in the drill bit it is anticipated that the reverse circulated fluid would compound the blockage at bottomhole and risk jamming the drill bit. Furthermore, this system requires the well to be on production which renders it ineffective for well intervention operations where production may in fact have halted.
In each of the disclosures described above, drilling cuttings are moved along the borehole during drilling; however there is no disclosure of how the cuttings may be removed from the wellbore fluid, or indeed from the well, in an efficient manner.
A drilling tool is described in DE 2808206 that comprises a rotatable cutting member, an impeller to circulate fluids and a separator that separates entrained material from the circulating fluid by means of a filter and store said entrained material in the tool. However, the applicant has realised that provision of an impeller and an opening to receive fluids with entrained drilling cuttings is not sufficient to ensure efficient cuttings capture.
WO 00/58602 discloses a cleaning tool used for cleaning casing-lined boreholes which includes a solids collection device, and U.S. Pat. No. 1,880,214 and U.S. Pat. No. 2,116,359 relate to drilling systems rotated from surface which include provisions for cuttings capture.
It is an object of aspects and/or embodiments of the present invention to provide a means for efficient collection, storage and/or removal of drilling cuttings when drilling or sidetracking wellbores, or drilling open hole laterals from a main wellbore in well intervention operations. Further aims and objects will become apparent from reading the following description.